This invention relates to an electron gun assembly for use in a cathode ray tube and more particularly to a multi-beam electron gun assembly for use in color television picture tubes.
Conventional color-reproducing cathode ray tubes include a multi-color image screen having interspersed groups of red-emitting, blue-emitting and green-emitting phosphor elements. Excitation of these elements is provided by an inline or delta cluster of three electron guns which emit three electron beams, each of which is focused into a beam spot on the tube screen by means of an electrostatic electron lens. The size of the electron spots focused on the screen, and thus the picture resolution, is a result of many factors. An important factor is the set of aberrations, particularly spherical aberration, introduced by the focusing lens. In the presence of spherical aberration, all electrons emanating from an object point do not, after focusing, recombine at a common point.
Commercially available electron guns for color cathode ray tubes have focusing lenses of two basic types. One type is the so-called "unipotential" type lens comprising three electrodes, the first and third of which are maintained at the same potential, typically the screen voltage, and a second (intermediate) of which is maintained at a much lower potential. The other type is the so-called "bipotential" lens comprising a relatively low voltage electrode followed by a second electrode which is maintained at a relatively high voltage, typically the phosphor screen voltage.
Designers of prior art focusing lenses have reduced spherical aberration by increasing the ratio of lens diameter to beam diameter. However, increasing lens diameter conflicts with the space limitations imposed by the neck diameters of standard color tube bulbs which are deliberately made small in order to minimize the yoke driving power required to deflect the beams, to minimize convergence power requirements and to minimize residual convergence errors. Neck size constraints are perhaps most severe in color tubes of the "small-neck" type having an "in-line" electron gun arrangement. For this arrangement, the maximum diameter of the focused lens for each electron beam must necessarily be less than one third of the neck inner diameter.
One way of reducing spherical aberrations without increasing lens diameter is to increase the length of the electrostatic lens in order to minimize electron beam bending at any one point. This can be accomplished by distributing the lensing action along the length of the gun. Among prior art lenses which make use of this approach are a double-Einzel lens disclosed in U.S. Pat. No. 3,863,091 to Hurakawa, et al.; a distributed Einzel lens disclosed in U.S. Pat. No. 3,895,253 to Schwartz et al.; a tripotential lens disclosed in U.S. Pat. No. 3,995,194 to Blacker, et al.; and a multi-element lens disclosed in U.S. Pat. No. 3,932,786 to Campbell.
As indicated in the Schwartz et al. patent, the double-Einzel concept does not appear to offer any distinct advantages over the distributed Einzel which replaces the high-low-high-low-high voltage distribution of the double-Einzel with a high-medium-low-medium-high voltage distribution and thus achieves an improved distribution of the fields along the axis of the lens. Both techniques suffer from a major practical disadvantage in that the high ultor potential, typically on the order of 25-30 kV, is brought very close to the low voltage end of the gun, thus increasing its vulnerability to electrical discharges.
The multi-element lens disclosed in the Campbell patent, although allowing a desired gradation of the fields, uses a relatively complex structure comprising a plurality of individual, electrically conducting plates mounted in spaced parallel relationship.
The tripotential lens disclosed in the Blacker et al. patent comprises four separate lens elements. The lens element closest to the cathode has an intermediate voltage applied thereto which, in the specific embodiment disclosed, is equal to 12kV. Although this voltage is less than the ultor voltage, it is still sufficiently high so as to present potential electrical discharge problems due to the proximity of the associated lens element to the low voltage end of the gun.